The present invention relates to a drop-in sensor measuring head having a forward facing bath thermocouple and a reverse-filling solidification chamber. The drop-in sensor according to the present invention allows for rapid and accurate measurements of the bath and liquidus temperatures of molten metals.
During the steelmaking process, oxygen is combined with carbon dissolved in the steel to remove the carbon, while at the same time providing heat from the combustion process. When the carbon content, as well as the molten metal temperature, are known, the furnace operator can decide the extent of additional refining or end the process. It is therefore advantageous to have available information regarding both the temperature of the liquid metal and the carbon content.
U.S. Pat. No. 3,559,452 describes the usefulness and terminology of carbon determination in molten steel during the converter process. It is a common practice to employ disposable immersion sensors, such as disclosed in U.S. Pat. No. 5,577,841, which are attached to a metal support lance and manually manipulated by an operator to obtain temperature and thermal analysis information. In many instances, ready access to the interior of steel processing vessels in order to contact the molten metal with disposable immersion measuring sensors, and thus determine the temperature and carbon content of the metal, is severely limited.
Large mechanical auxiliary lances, such as those described in U.S. Pat. No. 4,141,249, which are suitable for use in a converter process, have been constructed to immerse multifunctional sensor probes, such as those described in U.S. Patent Application Publication No. 2013/0098173, specially designed to attach to these auxiliary lances. However, there is a large capital expense required to modify an existing converter with an auxiliary lance. Once installed, the on-going maintenance of this equipment is difficult in the industrial environment of a typical steel mill increasing the cost per measurement. All of these expenses present a barrier to low cost steel production.
Drop-in or thrown-in devices have been developed to avoid the large capital investment of the auxiliary lance. Such measuring devices are dropped into the vessel from an access port well above the molten metal surface. These devices trail a signal cable that relays the thermocouple output to remote instrumentation providing data about the degree of processing accomplished, thereby enabling an operator to judge the extent of further processing required. Such devices for temperature measurement or thermal analysis are described in U.S. Pat. No. 3,357,598 and U.S. Pat. No. 3,463,005.
One difficulty in obtaining consistent and reliable molten metal sensor information from conventional drop-in devices is ensuring that the dropped sensor enters the liquid steel and remains submerged for a duration sufficient to obtain the desired measurements. Specifically, during the refining process, a large amount of slag floats on top of the molten steel and impedes the penetration of the dropped device through the slag layer and into the molten metal. If the displaced volume of the molten steel is significantly greater than the weight of the portion of the drop-in sensor designed to be immersed therein, the sensor will float rather than become immersed. The buoyant force acting on the body immersed in the molten steel will act in an upward direction, thereby resulting in a floating position partially or totally above the liquid steel.
It has been traditionally recognized that the random drop of these sensors could, in some circumstances, result in errors when the thermocouple or an opening to a thermal arrest chamber is not orientated for optimal contact with the molten steel. Both a side-extending thermocouple, such as disclosed in U.S. Pat. No. 3,574,598, or a side-opening thermal arrest chamber, such as disclosed in U.S. Pat. No. 3,463,005, could face away from the metal and thus experience errors due to limited molten metal contact, partial filling or not being filled at all as a result of floatation.
In CN 201041556, the opening to the thermal arrest chamber is positioned opposite the immersion end. The chamber is constructed of resin sand and is specified to be axially located. Since it has the liquid metal of the solidification chamber on its inner walls and its outer wall is directly exposed to the molten bath, it will have inefficient thermal capacity to chill the metal that has entered the inlet. Without complete thermal isolation of the solidification chamber, the molten metal bath can act as a heat pump, thereby heating the liquid phase of the solidifying mass in the chamber and resulting in liquidus measurement errors. These errors increase in number and magnitude as the molten metal superheat increases. In the conventional sense, superheat is determined by the difference of the temperature of the molten metal at the time of sampling over the temperature detected at the onset of its solidification. When a portion of the chamber exists outside of the main measuring head and is in thermal communication with the bath, it is subject to direct heating from the pool of molten metal which greatly diminishes the cooling ability of the chamber materials. The exit of the cable is secured to the metal head by a metal ring, but is not restricted in its lateral movements, which could destroy the resin sand solidification chamber in certain orientations.
To overcome this problem, improvements in these devices typically relied upon the bulk density of the device in relation to that of the liquid slag and that of the liquid steel, so as to aid in the descent of the device through the slag and into the steel, while providing some orientation toward the desired immersion direction. Bulk density is used herein to mean the overall density, including the sensor components and any voids within the overall immersed device and signal cable according to it submersed length. The multiple cross-sections in each configuration shown in U.S. Pat. No. 9,116,054 are graduated to accomplish a preferred orientation. That is, the total bulk density can be less than 7 g/cm3 as long as the a dense section of the measuring head has an apparent density greater than 7 g/cm3 necessary to ensure immersion of the measuring elements. Since liquid steel is approximately 90 percent as dense as solid steel, massive void free sections forward of the sensing elements are required to provide this orientation. The measuring elements are thus positioned at a distance removed from the forward face of the sensor. However, a sensing element at the forward face is the exact location that is favored for the longest exposure to the steel to be measured.
Still, at times, when the orientation is correct, that is, placing the temperature sensor in the metal and with a thermal arrest chamber that has filled with liquid metal, there exists a potential that accurate and reliable measurements are not obtained. U.S. Pat. No. 5,033,320 describes the necessary relation of the mass of the cooling chamber to the rate of solidification to obtain accurate thermal arrest information. While the bath temperature can be detected in a relatively short time, a longer time is required for the solidifying metal to reach a stable thermal balance between the released latent heat of fusion and the heat extraction of the cooling mass, and thus to obtain a liquidus plateau. From the time the drop in sensor is released, the trailing signal cable is constantly combusting. There is a potential for the destruction of the trailing signal cable to advance to a failed state before obtaining a longer duration liquidus plateau, such as that experienced for a high bath temperature, and thus failing to obtain the desired measurement. This is not new to the art. Various isolative and protective strategies have been employed in prior devices to delay the destruction of the signal cable by surrounding a portion of the signal cable exiting the measuring head with tubes of refractory material or cardboard, such as in U.S. Pat. No. 3,374,122.
Although this delay is temporary, it is suggested by U.S. Pat. No. 4,881,824 that when the measurement head has an adjustable paperboard tube float on its end facing away from the immersion end and the signal cable is guided through this tube, such an arrangement provides for sufficient protection of the signal cable to obtain the desired measurements. At the same time, the low density material also serves as a counterbalance, orientating the measuring head and stabilizing it in a down-facing position. Specifically, the temperature and sampling device of U.S. Pat. No. 4,881,824 does not have a solidification chamber but shows an “extension”. From the specification of U.S. Pat. No. 4,881,824, the low density float is adjustable along the length of the pipe to ensure that the proper immersion depth will be obtained with the sensor, and the float will maintain the probe in a generally erect position to avoid having the lead wires touch the surface of the molten metal which can damage the wires. The float keeps the pipe from tipping. As soon as the float encounters the melt or slag, it will tend to “right” the pipe. During immersion, the fusible link will melt and release the pod which will float up to the surface with the wire, allowing for retrieval of the sample mold.
However, in practice, the slag volume varies from heat to heat and during the steelmaking process, thereby eliminating any advantages or reason to have adjustable floats. This extension is not for the purpose of venting the measuring head. A separate hose is the vent for the sampler and is designed to separate from the measuring head. A simple comparison of the drawing of U.S. Pat. No. 4,881,824 (there are no dimensions in the specification) shows that metal parts are thicker than the float. Even if the float were weightless, the combination would not be less than the density of the slag whose value is underestimated in the specification. The metal parts have more volume than the measuring head by comparison.
Although the float should orient the measuring device in a sensor element downward position while providing protection for the trailing signals cable, this is only true in a mono-density fluid. In the typical steel making vessel with about 15% iron oxide contained in the slag at an intermediate process step, the density will be approximately 3 g/cm3, whereas the steel will have a density of nearly 7 g/cm3. Float-equipped devices have a forward bulk density that is still less than the liquid steel yet greater than the slag. Buoyant material attached to the opposite immersion end should orient the sensor within the first less dense liquid layer, but no quantity of float will force a lower density measuring head to submerse itself below the second higher density liquid.
It has been found that the falling momentum of the dropped sensor is paramount to positioning the measuring elements within the steel and filling the carbon chamber. Once the carbon chamber is filled, one must rely on this in-situ density, that is the measuring head density which includes a filled solidification chamber, to retard the sensor floatation for a time suitable to determine the bath temperature. A properly filled solidification chamber can deliver a correct liquidus measurement, even when partially submerged in the liquid metal.
Incremental improvements to drop-in measuring probes have eliminated many expected failure modes. However, many of these techniques are suitable only for drop-in devices utilized at the end of the converter process. This teaching is misleading. A measurement of the temperature and carbon content is most desired subsequent to the end of the process, namely during the blowing process. It is at this time that adjustments to the process could be effected. There are numerous conditions at an intermediate measurement time, that is during blowing, which escalate the difficultly of a drop-in sensor penetrating and passing into the metal at a suitable depth in order to acquire reliable measurements. With the exception of the few previously mentioned devices, most of the known prior art is absent of improvements which address the difficultly of using drop-in sensors for “in-blow” measurements.
Toward the end of decarburization, the amount of iron oxide transferred to the slag increases and thus reduces the viscosity, allowing for less drag, a reduction in slag volume due to the release of emulsified gases, and a leveling of the liquids at the end of blowing. All of these benefits are not available to those sensors dropped at in-blow. A large volume of slag that the sensor must pass through and a more viscous slag delay the exposure of the thermocouple to the steel and delay the opening of the solidification chamber. Previously employed floats can impede that passage of the measuring head through the slag. Delays in passing through the slag promotes freezing of the slag to the sensor's metal components, and thus increases the apparent volume and reduces bulk density. This, and the turbulence of the metal surface which is greatly increased during the blow, will reduce upright stability, with or without counterbalancing floats. Retention of the desired submerged position is highly compromised.
An objective of the present invention is to provide a cost efficient drop-in sensor for use in a steelmaking vessel during and prior to the end of the blowing process for providing a bath temperature and a thermal arrest temperature. More particularly, in order to correct the deficiencies of prior art devices and extend the usefulness of drop-in sensors to “in-blow” measurements, the present invention utilizes a simple construction to maximize the overall measuring head density. The measuring head according to the present invention also includes a solidification chamber which is thermally isolated from both the molten bath and the mass of the measuring head, thereby resulting in an improved ratio of cooling mass to sampled mass and thus promoting a reduction of the time to achieve a stable liquidus plateau. All external surfaces of the measurement head are designed to maximize the falling momentum and all internal spaces are maximized to minimize the buoyant tendency of the measurement head once submersed. The measurement head of the present invention also does not utilize a float.